Our aim is to gain a better understanding of the molecular basis for the biological roles of lactoferrin, the major iron-binding protein in milk, in many other bodily secretions, and in leukocytes. This will address its antibacterial properties, as well as mechanisms of uptake and release o metal ions. It will have important implications for understanding bodily defence mechanisms and in particular the biology of human milk and the bioavailability of trace elements. (i) Specifically, the three-dimensional structure of human lactoferrin will be determined at high resolution by X-ray crystallography. The structure will then be refined, so that the exact nature of the iron sites, their location and accessibility in the protein structure, and the role of functionally important anions (HCO3 ion) can be unambiguously determined. The high resolution analysis will build upon preliminary studies already carried out. Large crystals of lactoferrin, which are stable to X-rays, have been obtained; heavy atom derivatives have been prepared and characterised; and X-ray studies at 6A and 3.5A have given promising results. (ii) Once the high resolution structure has been determined, binding studies will be carried out with other metal ions (eg Zn2+, Cu2+, Ca2+, Cr3+, Mn2+) and anions (eg oxalate) which may bind either in the specific iron sites or in non-specific sites. This will help define the role of lactoferrin in binding trace element in milk, and in influencing their bioavailability. (iii) The structure will also be used to examine structural and functional relationships with other iron-binding proteins, notably serum transferrin, the iron transport protein in plasma, and ovotransferrin. (iv) Apolactoferrin, iron-binding fragments of lactoferrin, and lactoferrins of other species (e.g. bovine) will be examined in comparative studies. (v) As a long-term aim, the structure will be examined through site-directed mutagenesis studies presently being initiated by other researchers.